The present invention relates to a color video camera, especially a color video camera having a high sensitivity.
Recently, color video cameras using solid state image pick-up devices having a two dimensional array of photo-electric elements have become popular as a result of the popularization of video cassette recorders (VCRs). It is well known that a complementary filter for transmitting colors: cyan (Cy), yellow (Ye) or magenta (Ma), can be utilized as a color filter in the color video camera to increase the sensitivity of color video cameras using solid state photo-sensitive devices. For example, such filters are described in Nabeyama et al.: All solid state color camera with single-chip MOS imager: IEEE Tr. on CE, Vol. CE. 27 (Feb., 1981), Ohba et al.: MOS Imaging Random Noise Suppression: ISSCC Digest, P26 (Feb., 1984), or Sato et al.: U.S. Pat. No. 4,246,601 (issued on Jan. 20, 1981).
If red (R), Green (G) and Blue (B) are considered to be three primary colors, then Cy is a complementary color to R, that is, R and Cy are mixed to form white (W). Ma and Ye are the complementary colors to G and B, respectively. The complementary filter described in this specification can have separate color filter elements for transmitting each of the colors Cy, Ye and Ma, or a color filter comprising the color filter elements. Also, in this specification,, a primary color filter can have separate color filter element for transmitting each of the colors R, G and B, or a color filter comprising the separate color filter elements. When a complementary filter is utilized, the luminance S/N ratio becomes about 6 dB higher than when a primary color filter is used.
The cause will be described hereinafter. FIG. 1a illustrates an explanatory view showing the arrangement of a primary color filter as the color filter of the color video camera, and FIG. 1b similarly illustrates the arrangement of a complementary filter. In this Figure, G, R, B, W, Cy and Ye respectively denote green, red, blue, transparent (white), cyan and yellow color filters, and also denote image signals for one picture element.
A luminance signal y.sub.p of an image or an object is provided by adding the signals for four picture elements transmitted through the four filters indicated in FIG. 1a and the following formula is derived; EQU y.sub.p =B+2G+R (1)
Similar to the above, a luminance signal y.sub.c of the image is provided by adding the signals for four picture elements transmitted through the four filters indicated in FIG. 1b and the following formula is derived; ##EQU1##
Moreover, there is no need to repeatedly indicate that the W filter can pass R, G and B and that the C.sub.y filter can pass B and G and Y.sub.e can pass R and G.
Comparing formula (1) with (2), it is evident that the received image signal quantity can be doubled when complementary filters are employed.
When the complementary filter arrangement indicated in FIG. 1b is employed, a red channel signal r.sub.c can be derived from the following formula; EQU r.sub.c =W-G-C.sub.y +Y.sub.e =2R (3)
Similarly, a blue channel signal B.sub.c can be derived from the following formula; EQU B.sub.c =W-G+C.sub.y -Y.sub.e =2B (4)
In the primary color filter arrangement indicated in FIG. 1a, a red channel signal r.sub.p and a blue channel signal b.sub.p respectively have relations of r.sub.p =R and b.sub.p =B. The following formulas are derived: EQU r.sub.c =2r.sub.p ( 5) EQU B.sub.c =2b.sub.p ( 6)
These formulas (5) and (6) indicate that the signal quantity of the color channel signals can be doubled when complementary filters are employed, compared with employing primary color filters.
Next, the noise for each color channel will be studied. FIG. 2 illustrates a schematic explanatory view showing random noise in the four filters, more exactly, corresponding to the four picture elements.
In the case of the previously cited prior art of Nabeyama et al., each of the four color filters provides an independent readout signal when converting the image transmitted through the color filters into an electrical signal and then it is read as the amplitude of the electric charge. Thus, there is no relation among the noise signals nl to n4 in the respective filters.
When employing the complementary filters shown in FIG. 1b as the color filter, a random noise n.sub.yc of the luminance signal, a random noise n.sub.rc of the red channel signal and a random noise n.sub.bc of the blue channel signal are respectively expressed as follows; EQU n.sub.yc =n1+n2+n3+n4 (7) EQU n.sub.rc =n1-n2-n3+n4 (8) EQU n.sub.bc =n1-n2+n3-n4 (9)
It is evident that these formulas (7) to (9) can be derived by referring to the above-stated formulas (2) to (4).
Next, in the case of employing a primary filter such as indicated in FIG. 1a as the color filter, random noise n.sub.yp of the luminance signal, random noise n.sub.rp of the red channel signal and random noise n.sub.bp of the blue channel signal are respectively expressed as follows; EQU n.sub.yp =n1+n2+n3+n4 (10) EQU n.sub.rp =n2 (11) EQU n.sub.bp =n3 (12)
As indicated above, it is evident that random noise in the color channel signal is doubled, that is, its power is quadrupled, when employing the complementary filter compared with employing the primary color filter. Considering that the signal strength is doubled by employing a complementary filter, S/N ratios are improved to 6 dB in the luminance signal and 0 dB in the color channel signals by the complementary filter. The improvement of the color S/N ratios in the color video camera utilizing the complementary filter is deemed to be effectual in improving the sensitivity of the camera.
Next, we will consider the case where there is a relationship between the random noise values in the filters, that is, the picture elements.
The above-mentioned prior art by Ohba et al. is designed to use a common read-out signal, that is, one signal line extending vertically used for two vertically adjacent picture elements of a two dimensional array of picture elements, that is, photo electric elements, for example, W and C.sub.y. Since random noise is mainly caused when the charges are transferred from the signal line to a horizontal transfer section, there is a correlation for random noise between the vertically adjacent picture elements.
The signals corresponding to FIG. 3 schematically show the correlation. In FIG. 3, nl' denotes the noise on the vertical signal line caused after unnecessary signals such as a vertical smear signal are swept away from the vertical signal line to the outside prior to reading out the signal W, n2' denotes the noise on the vertical signal line caused after the signal W was transferred to the horizontal transfer section, n3' denotes the noise on the vertical signal line caused after the signal C.sub.y was transferred to the horizontal transfer section, and n4' to n6' denote the noise on a next vertical signal line, corresponding to G and Ye, respectively. In this instance, as indicated in FIG. 3, the noise of W and C.sub.y is (n1'-n2') and (n2'-n3'), respectively, and the correlation between them is established.
The random noise for respective signals is given by the following formulas; EQU n'.sub.yc =n1'-n3'+n4'-n6' (13) EQU n'.sub.rc =n1'-2n2'+n3'-n4'+2n5'-n6' (14) EQU n'.sub.bc =n1'-n3'-n4'+n6' (15)
Thus, it is understood that the noise n'.sub.rc of the red channel signal has three times the power of n.sub.bc of the blue channel signal, that is, greater by about 5 dB.
Camera sensitivity can be improved by improvement of the S/N ratio of the red channel signal in the color filter arrangement indicated in FIG. 1b. If Cy is replaced with Y.sub.e, the noise n.sub.bc of the blue channel signal is greater than that of the red channel signal. However, this color filter arrangement is disadvantageous because of moire in the luminance signal, and the filter arrangement indicated in FIG. 1b is usually employed.
As mentioned above, in a color video camera using a solid state image pick-up device, it will be effective to improve the S/N ratio of the red channel signal in order to increase the sensitivity. For that purpose, an infrared cut-off filter having a higher cut-off characteristic than the prior art can be used. However, that would raise another problem mentioned hereinafter.
FIG. 4 shows a graph of spectral sensitivity of the solid state image pick-up elements formed on a silicon (Si) chip. This graph is a large scale linear-approximation for simplification. The data is measured by attaching the solid state image pick-up elements (MOS transistor-type) used in the aforementioned prior art to the complementary filter indicated in FIG. 1b and then using a 3200.degree. K. halogen lamp and a beryl filter. In this graph, the abscissa denotes wavelength and the ordinate denotes relative sensitivity. In this Figure, the real line denotes the sensitivity corresponding to W and the dotted line denotes the sensitivity corresponding to G. The characteristic traced by the dotted line on the left and the solid line on the right denotes the sensitivity corresponding to Ye. The characteristic traced by the solid line on the left and the dotted line on the right denotes the sensitivity corresponding to Cy.
The spectral sensitivity characteristics of the luminance signal y, the red channel signal r, and the blue channel signal b can be obtained by substituting the spectral sensitivity characteristics indicated in FIG. 4 into the above-mentioned formulas (2) to (4). The result obtained thereby is shown in FIG. 5.
In this Figure, the sensitivity corresponding to y is denoted by a solid line; the sensitivity corresponding to r is denoted by a dotted line on the left and a solid line on the right; and the sensitivity corresponding to b is denoted by a solid line on the left and a dotted line on the right.
In the conventional prior art, the spectral sensitivity characteristics of y, r and b, which correspond approximately to the standard color specification system defined by CIE (Commission Internationale de l'Echairage (International Commission of Illumination)), are obtained by cutting off the wavelength y.sub.c, which is over 630 to 650 nm, with a near infrared light cut-off filter. It is necessary to standardize the spectral sensitivity indicated in FIG. 5 by dividing the spectral components of the halogen lamp so as to compare said spectral sensitivity with stimulus values of the standard color specification system.
The examples of spectral sensitivity characteristics including conventional near infrared cut-off filters are described in Takemura et al.: CCD 2-plate-style color television camera: TV Scientific Society of Japan, Vol. 33, Section 7, page 28 (1979). Also, in the prior art, for example, T. Inai et al., U.S. Pat. No. 4,437,111 (issued on Mar. 13, 1984), a color video camera using a pick-up tube and an infrared cut-off filter is disclosed.
Therefore, it is clear that to increase the sensitivity of a color video camera, especially a one using a solid state image pick-up device, the S/N ratio of the red channel signal must be improved by using an infrared cut-off filter having a higher cut-off frequency. It is also clear that the spectral sensitivity of the color video camera will deviate from the standard color specification system defined by CIE when the S/N ratio of the red channel signal is thus improved.